The present invention relates to a duplex stainless steel alloy composition, and more particularly to a copper-bearing duplex stainless steel alloy composition, which has exceptional pitting resistance.
The alloy of the present invention has useful applications in the fields of chemical industry and pulp and paper manufacturing industry. The alloy can be used in such applications as vessels, retorts and piping and for paper machine roll shells for non-suction roll applications such as coater rolls, grooved rolls, and blind-drilled rolls and for suction roll applications such as breast rolls, couch rolls, pickup rolls, press rolls and wringer rolls.
The use of copper in austentic stainless steels, such as Carpenter Alloy 20 and CN-7M, and in duplex stainless steels, such as CD-4MCu (U.S. Pat. No. 3,082,082) and Ferralium Alloy 255 (U.S. Pat. No. 3,567,434) is well-known. The CD-4MCu and Ferralium Alloy 255 alloys are duplex stainless steels that were developed as casting alloys, and contain about equal amounts of austenite and ferrite. The CD-4MCu alloy and the Ferralium 255 alloy are similar to the Alloy 75 composition produced by the Sandusky Foundry and Machine Company. The nominal chemical composition of the three alloys are as follows:
______________________________________ Chemical Composition, percent Alloy C Cr Ni Mo Cu N ______________________________________ CD-4MCu 0.04 25.5 5.5 2.0 3.0 -- Ferralium Alloy 255 0.04 25.5 5.5 3.0 1.7 0.17 Alloy 75 0.03 26 6.8 -- -- -- ______________________________________
It can be seen from the above tabulation that CD-4MCu and Ferralium Alloy 255 are very similar. A significant difference is that Ferralium Alloy 255 contains an intentional nitrogen addition. Both the CD-4MCu and Ferralium alloys contain 2 percent or more molybdenum. The addition of molybdenum improves the pitting resistance of stainless steel in chloride-containing environments. An empirical pitting index is employed to predict the pitting and crevice corrosion resistance of a stainless steel based upon its chemical composition. The pitting index is determined by measuring the chromium content plus three to four times the molybdenum content. The higher the pitting index value, the better the pitting resistance. Molybdenum, being a strong ferrite promoter, tends to concentrate in the ferrite phase in duplex stainless steels, therefore the austenite phase may contain less than half the molybdenum content of the ferrite. Molybdenum also fosters the formation of signma and chi phases within the ferrite during slow cooling through, or exposure in, the range from about 1700.degree. F. to 1100.degree. F. Molybdenum also hastens the formation of the alpha prime phase in the ferrite in the range from 1000.degree. F. to 700.degree. F. Both sigma, chi and alpha prime phases reduce very significantly the ductility and toughness of stainless steel. Thus, to obtain good mechanical properties, molybdenum-containing duplex stainless steels must be rapidly cooled from the solution annealing temperature. In the prior art alloys copper is added to contribute precipitation hardening capabilities. An aging treatment at 900.degree. F. for 2 hours will increase the yield and tensile strengths about 15 to 20 percent. That aging treatment is no longer recommended for the CD-4MCu alloy.
The duplex stainless steels have certain advantages over the fully austenitic stainless steels. The duplex steels have much higher yield and tensile strengths, and are not as susceptible to sensitization, intergranular corrosion, and intergranular stress corrosion cracking. Alloy 75 was developed for suction roll shell applications to take advantage of those attributes. In contrast to the molybdenum-containing duplex stainless steels, Alloy 75 can be slowly furnance cooled from a high temperature without fear of excessive formation of brittle phases. In addition, furnace cooling results in a very low level of residual stress.
High tensile residual stresses are very detrimental to the service performance of suction rolls employed in paper making machines. The molybdenum-bearing duplex stainless steels (such as Alloy A171, Alloy 63, CD-4MCu and Ferralium Alloy 255) which must be rapidly cooled from the solution-annealing temperature, will have very high levels of tensile residual stresses which are very detrimental to service performance. For example, Alloy 63, a modified CF-8M alloy containing about 30 percent ferrite that has exceptional corrosion resistance and very high corrosion fatigue strength, has given poor service in paper machines. The high level of the tensile residual stresses in rapidly cooled shells has lead to premature corrosion fatigue failures.
Although furnace cooling of Alloy 75 shells has led to very low levels of residual stress and good service performance, Alloy 75 lacks the pitting resistance of the molybdenum-bearing stainless steels in highly corrosive environments. In most paper mill white waters, Alloy 75 has adequate pitting resistance. However, Alloy 75 can pit when corrosive conditions become very severe. For instance, when mills close up the white water system, the chloride ion concentration increases and a species of sulfur compound, the thiosulfate ion, can build up in the white water.
Pitting of Alloy-75 rolls has occurred in paper mill service in environments containing high levels of the chloride and thiosulfate ions. Alloy 75 has also been found to pit in laboratory tests in similar environments. Pitting has been found to occur in the austenite and at austenite/ferrite interfaces. Pitting of the ferrite phase has not been detected. Energy dispersive X-ray analysis has shown that the chemical composition of the ferrite and austenite in Alloy 75 is about as follows:
______________________________________ Chemical Composition, percent Cr Ni ______________________________________ Austenite 22 10 Ferrite 31 5 ______________________________________
The relatively low chromium content of the austenite phase is believed to be responsible for its reduced pitting resistance.
Accordingly, an essential object of the invention is to improve the pitting resistance of duplex stainless steels.
The objects and advantages of the invention will be apparent to those skilled in the art from a reading of the present specification and claims.